Plant protein materials are used as functional food ingredients, and have numerous applications in enhancing desirable characteristics in food products. Soy protein materials, in particular, have seen extensive use as functional food ingredients. Soy protein materials are used as an emulsifier in meats, including frankfurters, sausages, bologna, ground and minced meats and meat patties, to bind the meat and give the meat a good texture and a firm bite. Another common application for soy protein materials as functional food ingredients is in creamed soups, gravies, and yogurts where the soy protein material acts as a thickening agent and provides a creamy viscosity to the food product. Soy protein materials are also used as functional food ingredients in numerous other food products such as dips, dairy products, tuna, breads, cakes, macaroni, confections, whipped toppings, baked goods and many other applications.
In general, soy protein materials include soy flakes, soy flour, soy grits, soy meal, soy protein concentrates, soy protein isolates, and mixtures thereof. The primary difference between these soy protein materials is the degree of refinement relative to whole soybeans. Soy flakes are generally produced by dehulling, defatting, and grinding the soybean and typically contain less than 65 wt. % soy protein on a moisture-free basis. Soy flakes also contain soluble carbohydrates, insoluble carbohydrates such as soy fiber, and fat inherent in soy. Soy flakes may be defatted, for example, by extraction with hexane. Soy flours, soy grits, and soy meals are produced from soy flakes by comminuting the flakes in grinding and milling equipment such as a hammer mill or an air jet mill to a desired particle size. The comminuted materials are typically heat treated with dry heat or steamed with moist heat to “toast” the ground flakes and inactivate anti-nutritional elements present in soy such as trypsin inhibitors. Heat treating the ground flakes in the presence of significant amounts of water is avoided to prevent denaturation of the soy protein in the material and to avoid costs involved in the addition and removal of water from the soy material. The resulting ground, heat treated material is a soy flour, soy grit, or a soy meal, depending on the average particle size of the material. Soy flour generally has a particle size of less than 150 μm. Soy grits generally have a particle size of 150 to 1000 μm. Soy meal generally has a particle size of greater than 1000 μm.
Soy protein concentrates typically contain 65 wt. % to 90 wt. % soy protein, with the major non-protein component being fiber. Soy protein concentrates are typically formed from defatted soy flakes by washing the flakes with either an aqueous alcohol solution or an acidic aqueous solution to remove the soluble carbohydrates from the protein and fiber. On a commercial scale, considerable costs are incurred with the handling and disposing of the resulting waste stream.
Soy protein isolates, more highly refined soy protein materials, are processed to contain at least 90% soy protein and little or no soluble carbohydrates or fiber. Soy protein isolates are typically formed by extracting soy protein and water soluble carbohydrates from defatted soy flakes or soy flour with an alkaline aqueous extractant. The aqueous extract, along with the soluble protein and soluble carbohydrates, is separated from materials that are insoluble in the extract, mainly fiber. The extract is typically then treated with an acid to adjust the pH of the extract to the isoelectric point of the protein to precipitate the protein from the extract. The precipitated protein is separated from the extract, which retains the soluble carbohydrates, and is dried after being adjusted to a neutral pH or is dried without any pH adjustment. On a commercial scale, these steps contribute significant cost to the product.
In addition to the soy protein content, flavor, odor, and color of a soy protein material are also a relevant criteria for the selection of a soy protein material as a functional food ingredient. Conventional soy protein material may have a strong beany, bitter flavor and odor as a result of the presence of certain volatile compounds (e.g., hexanal, diacetyl, pentanal, n-pentane and octanal) and/or an undesired appearance due to the presence of other relatively low molecular weight compounds (e.g., isoflavones) in the soy protein material.